I just ran some application which maybe have memory leakage issues on my box with debian 6.01a installed. It consumed about 5GB of the total 8GB RAM and didn't release the RAM when it exited. So I rebooted my machine, to get my memory back. However, I found it didn't. When the machine started again, both the top command and the free command show that there is still 5GB RAM occupied. It's really unstandable to me, so is there anyone could explain why this happens?

Update:
Finally, i found it was not a problem about the system's memory management. The reason why lots of RAM is consumed is that the app wrote lots of data - about 5GB - to the redis server, and that's why when the system started, about 5GB RAM was occupied. Sorry for this misleading question.